The inventors have proposed a yarn feeding device for feeding a yarn of required length using a servomotor in accordance with a knitting section to be knitted, and supplying the yarn to a carrier of a knitting machine by means of an arm functioning as a buffer (Patent Document 1: JP4016030B, Patent Document 2: JP 2006-169675A). The amount of yarn required per unit of time is referred to as “yarn speed” in this specification. The yarn speed is decided by a yarn length used for forming stitches and changes in the yarn length between the buffer arm and the needle bed that are caused by a motion of the carrier. The buffer arm is often simply called “arm.”
According to Patent Documents 1 and 2, the arm is biased by a spring so as to provide a substantially constant tension to the yarn. Here, Patent Document 1 discloses that the loop length of the stitch for each knitting needle is calculated based on the knitting data and that the knitting yarn just required for knitting is fed out actively in synchronization with a motion of the carrier. When knitting at high speed, however, it has been found that simply controlling the amount of yarn to be fed might cause a high tension peak to the yarn and consequently cut the yarn. Patent Document 2 discloses that a yarn feeding speed is increased prior to a sharp increase of the yarn speed to store excess yarn in the arm in order to prepare for a section where the yarn speed increases. However, when a sufficient amount of the excess yarn is reeled out beforehand, the yarn tension lowers, and consequently the yarn becomes loose. For these reasons, it is difficult to reduce the yarn tension fluctuations when knitting at speeds higher than the speeds assumed in Patent Documents 1 and 2.
The present inventors, therefore, examined to actively control the arm torque by means of a torque generator such as a torque motor, instead of passively controlling the arm by means of the spring. With regard to controlling the torque of the arm of a flat-knitting machine, a circular knitting machine or other knitting machines, Patent Document 3: JP2951068B discloses performing feedback control on the drive torque of the arm by means of a tension sensor provided on the downstream side of the buffer arm. However, according to the experiments by the present inventors, the feedback control has been found not enough to prevent the high tension peak at high speed knitting. The yarn tension peak caused small stitches where the loop lengths thereof were reduced, and sometimes the yarns were cut because the tension of the yarns exceeded their durability. Even at the conventional knitting speeds, decorative yarns or the like easily broken yarns may be cut with the fluctuations in the yarn tension.    Patent Document 1: JP4016030B    Patent Document 2: JP2006-169675A    Patent Document 3: JP2951068B